Return-to-zero (RZ) and short pulse (quasi-soliton) type data formats are currently receiving increased attention from optical telecommunication system designers, because of their inherent advantage over non-return-to-zero (NRZ) formats in a number of high bit rate, laser-based communication applications, such as those operating at very high data rates (e.g., on the order of ten to several tens of Gb/s or higher). For example, RZ (and RZ-type) data formats have been shown to allow increased peak power transmission over extended span lengths by minimizing the effects of chromatic dispersion.
Although an NRZ optical transmitter typically employs only a single electro-optic modulator, the hardware for implementing an RZ external transmitter (such as for long haul, high bit rate fiber communications) is more complex, as it may require two cascaded electro-optic modulators—one for RZ signals, the other for NRZ signals. For proper transmitter operation, in addition to the separate bias control for the RZ and NRZ modulators, there must be some way of maintaining phase alignment between the RZ and NRZ signals, irrespective of changes in ambient operational parameters, e.g., aging and temperature.